Respiration during sleep will be measured in micro-gravity by instrumenting subjects with a Respiratory Inductance Plethysmograph (RIP) and pulse oximeter allowing continuous measurement of the motion of both the rib cage and abdomen and arterial oxygen saturation. In addition, subjects will be fitted with an EEG, EOG, an ingestible body temperature sensor, and with an EC allowing the Principal Investigator and his team to determine sleep stages. From these sensors, they can determine changes in ventilation, rib cage and abdominal contribution to ventilation, thoraco-abdominal asynchrony, sympathetic and parasympathetic contributions to heart rate variability, and the coupling between respiration and heart rate, all as a function of sleep stage. In addition, the Principal Investigator and his team will study the neurological control of ventilation by measuring the ventilatory response to both hypoxia and hypercapnia. In-flight measurements will be made of the quasi-isocapnic hypoxic response and hypercapnic rebreathing response. In addition, cardiac output diffusing capacity, lung water, and resting oxygen consumption will be measured. These will be supplemented by RIP and pulse oximetry measurements allowing determination of respiratory timing without the interference of a mouthpiece and arterial oxygen saturation. Pre- and post-flight, the same measurements will be made as well as carefully controlled ventilatory response tests, and carotid baroreceptor-cardiac reflex measurements. These data will provide the Principal Investigator and his team with information regarding the change in ventilatory control and ventilatory-baroreceptor-integrated reflex. The combination of sleep studies and the awake measurements performed on the same subjects in micro-gravity will shed considerable light on the changes in the neurologic control of ventilation that occur when gravity is removed.